1. Field of the Invention
The present invention relates to a speaker and a speaker apparatus.
2. Description of the Related Art
Electrodynamic, electrostatic, piezoelectric, electromagnetic and other types of speakers are conventionally known. Electrodynamic speakers (dynamic speakers), which are classified into a moving-coil type, a ribbon type, a Blatthaller type, a Heil type, etc. in terms of the construction of diaphragm and vibration source, are mainstream speakers since they are basically designed to drive a diaphragm by an electromagnetic force and achieve a high conversion efficiency with a relatively simple construction. A large majority of electrodynamic speakers are of moving-coil type such as a cone speaker and a dome speaker.
As shown in FIG. 16, the cone speaker includes a conical diaphragm (cone diaphragm C), a voice coil Vc and a magnetic circuit Mc that generate an electromagnetic force for driving the cone diaphragm C, a damper D that holds the voice coil Vc at a constant position relative to a gap Gp in the magnetic circuit Mc, an edge E that supports the periphery of the cone diaphragm C coupled to a tip end of the voice coil Vc, and a frame F that couples the above component parts into one piece.
To drive the cone diaphragm C, an electric current is caused to flow through the voice coil Vc disposed in the gap of the magnetic circuit Mc in accordance with an audio waveform, whereby the voice coil Vc directly coupled to the cone diaphragm C is reciprocated in a direction perpendicular to the line of magnetic force.
In the cone speaker, the damper D and the edge E serve to hold the voice coil Vc at the center of vibration. When using such a mechanical spring construction to hold the voice coil at the vibration center, the vibration of diaphragm C largely depends on the mechanical characteristic of the mechanical spring construction. Thus, a frequency-dependent variation is caused in sound emission, posing a problem. Another problem is that there is a limit in increasing the amplitude of vibration of diaphragm C.
To eliminate the problems of the mechanical support construction, the below-mentioned techniques have been proposed.
A method for solving the problems by using an air damper instead of the mechanical support construction is disclosed in the following document.
Guy Lemarquand “New structure of loudspeaker”, Audio Engineering Society Convention Paper 6846 presented at the 120th Convention, May 2006
Another technique is disclosed in Japanese Laid-open Patent Publication No. 2001-186589, in which a bias current is added to an audio signal-based driving current for each coil of a double coil construction for adjustment of the vibration reference position.
Still another technical art disclosed in Japanese Laid-open Patent Publication No. 11-164394 includes a braking coil for exerting a braking force on the voice coils, the braking coil being disposed outside magnetic fields when the voice coils are each at a stationary position. In this technique, the vibration reference position is adjusted by a conventional mechanical structure.
In the technique disclosed in the document entitled “New structure of loudspeaker”, the mechanical characteristic corresponding to a spring constant is determined by the air damper structure, thus posing a problem that the degree of freedom of design is limited. In addition, an air chamber is required, which is large in volume in comparison with its diameter.
In the technique disclosed in Japanese Laid-open Patent Publication No. 2001-186589, an audio signal is supplied to the coils for being held in the vibration center position. When the coils are each at a position other than the vibration center position, these coils are deviated from a magnetic gap region, posing a problem that the efficiency of utilizing the magnetic field is lowered. Other problems are that the coils must be applied with biases which are opposite in polarity, resulting in a complicated driving circuit, and these coils are difficult to be utilized for any purpose (such as a motion feedback sensor) other than intended one.
In the technique disclosed in Japanese Laid-open Patent Publication No. 11-164394, when the voice coils are at vibration center positions, the braking coil must be positioned at a position deviated from the magnetic field. In such a state, a Lorentz force for realizing a damperless structure cannot be obtained.